Primary electrochemical cells are generally well known and available in a variety of sizes and shapes. One well known form of a primary electrochemical cell is a cylindrical cell including a battery stack disposed within a generally cylindrical metal housing and exposed to and permeated by an electrolytic solution. U.S. Pat. No. 4,060,668, in the name of Franz Goebel and assigned to GTE Laboratories Incorporated, discloses a cylindrical primary electrochemical cell having a battery stack including a lithium anode, a carbon current collector cathode structure, and a separator of insulative material interposed between the lithium anode and the carbon current collector cathode structure. The lithium anode takes the form of a cylindrical sheet physically pressed against an interior wall of the metal housing, and is arranged concentrically within the housing with the carbon current collector cathode structure and the separator. A preferred electrolytic solution for the above-described cell includes a reducible soluble cathode and an electrolyte solute dissolved in the reducible soluble cathode. Suitable materials for the reducible soluble cathode and the electrolyte solute are thionyl chloride and lithium tetrachloroaluminate, respectively.
In co-pending application Ser. No. 333,094, filed Dec. 21, 1981, in the name of Franz Goebel, entitled "ELECTROCHEMICAL CELL" and assigned to the same assignee as the present application, an alternative construction of a cylindrical cell is disclosed in which a cylindrical, generally-solid lithium anode is disposed centrally within a cylindrical metal housing of the cell and surrounded, in turn, by a separator of insulative material and a carbon cathode structure. These components of the cell are also exposed to an electrolytic solution of the same type and composition as disclosed in U.S. Pat. No. 4,060,668 briefly discussed hereinabove.
While both of the cylindrical cells as described hereinabove operate in a satisfactory manner, neither has the capability of providing a warning signal, indication, or other condition toward the end of discharge of the cell indicating that the cell is approaching the end of its useful or rated life. Such an indication may be useful, for example, in applications in which an early and timely replacement of the cell is necessary or desirable.
Cells capable of providing end-of-life warning indications as described above are disclosed in U.S. Pat. Nos. 4,247,607 and 4,293,622. In U.S. Pat. No. 4,247,607, a disc or button-type lithium halide primary cell is disclosed including a lithium anode in contact with an iodine/polymer depolarizer and having a step portion comprising about 15 to 5% of the total thickness of the anode and a surface area of from about 40 to 60% of the surface area of the major surface in contact with the depolarizer. An indication of approaching end of life is provided by a substantial, detectable increase in the internal impedance of the cell when only 5 to 15% of the unused lithium anode remains. In U.S. Pat. No. 4,293,622, a lithium/thionyl chloride cell is disclosed in which the electrochemically active components of the cell, including the electrolytic solution, are selected so that the electrolytic solution is exhausted during discharge before the other active components. As a result, a detectable step or transition occurs in the output voltage of the cell which serves as an indication of impending cell discharge. A disadvantage of the cell design of this latter patent is that the limiting of the amount of electrolytic solution makes less of the electrolytic solution available for absorbing soluble gaseous discharge products, such as sulfur dioxide, with the result that an undesirable increase in internal cell pressure may occur.